1. Field of the Invention
The present invention relates to a burner for operating a heat generator.
2. Discussion of Background
It is known from the literature that, in the case of a perfectly premixed flame, the size of the flame-stabilization zone, also known by the term backflow zone, has no effect on the NOx emissions. However, the CO and UHC emissions and especially the extinction limits are greatly affected. This means that, the larger the flame-stabilization zone, the lower the CO and UHC emissions and the extinction limit. With a larger stabilization zone, a larger load range of the burner can therefore be covered in premix operation without the flame being extinguished.
EP-0 321 809 B1 discloses a premix burner which is based on the generation of an enclosed swirl flow in the cone head, which swirl flow, on account of the increasing swirl along the cone tip, merges into an annular swirl flow with backflow in the core. The location where this breakdown of the flow takes place is determined by the cone angle and the inflow ducts for directing a combustion-air flow into the interior space of the swirl generator. The size and the general configuration of this backflow zone or backflow bubble (vortex breakdown) are thereby also defined.
EP-0 780 629 A2 discloses a further premix burner in which measures are taken in order to shift the backflow bubble further downstream, this in order to obtain a longer premix and vaporization section. For this purpose, a mixing tube is arranged downstream of a swirl generator, which acts on the head side of the premix burner and is based here on the premix burner according to EP-0 312 809 B1, transition geometry, which consists of transition passages for passing the swirl flow from the swirl generator into the mixing tube without separation, being arranged intermediately between swirl generator and mixing tube. These transition passages are disposed in sectors, in accordance with the number of inflow ducts acting in the swirl generator. This configuration inevitably reduces the size of the backflow zone, since the swirl of the flow has to be selected in such a way that the flow does not break down inside the mixing tube. At the end of the mixing tube, the swirl is therefore rather small for a large backflow zone to develop. If it is attempted to intensify this backflow zone with a larger diffuser angle of the mixing tube, problems arise in the regions of the diffuser which are close to the wall (boundary layers, separations) and the flame then migrates easily upstream. The configuration of the burner outlet at the end of the mixing tube with a breakaway edge has produced a significant improvement with regard to intensification of the flame stability, lower pollutant emissions, lower pulsations, complete burn-out, large operating range, good cross-ignition between the various burners, compact type of construction, improved mixing, etc. However, it has been found that a further intensification of the flame stability as well as improved adaptation of the flame to the predetermined geometry of the combustion chamber is necessary for smooth operation at the highest level in the premix combustion of the newer generation.